Undesirable solutes in liquid runoff can originate from a variety of sources, including atmospheric deposition and mobilization of substances occurring on or near a ground surface. Methods to reduce the amount of unwanted solutes in runoff streams include the construction of catchment basins wherein the runoff stream accumulates until it either evaporates or permeates into the ground; grass filter strips wherein the runoff stream's velocity is reduced allowing infiltration; drainage systems incorporating buried permeable reactive barriers wherein runoff drainage is directed through a subsurface packed media bed that immobilizes the solutes as the drainage passes through; and aboveground permeable reactive barriers comprising large three dimensional structures the size of the area to be treated constructed on drainage-way sites and filled with reactive media.
These present methods are limited in application because they either fail to adequately treat the liquid runoff or they have installation and maintenance requirements that are not suitable for many settings. Catchment basins require a large space that may not be available, particularly at established facilities. Grass filter strips may also require a large space and may be less effective on steep slopes or during high rainfall events. Buried permeable reactive barriers are difficult to access when maintenance is required and may require an additional drainage system to direct liquid runoff to and from the barrier, which increases the cost and complexity of installation. Aboveground permeable reactive barriers constructed by filling a large structure with loose reactive media require special skills and bulk material handling equipment to construct and may be difficult to maintain, partially replace, or augment. Also, it is very difficult to use existing methods to construct a barrier designed to remove several different solutes from liquid runoff by using multiple media types that must be kept separate.
Another method that could conceivably be used to reduce the amount of undesirable solutes in liquid runoff is discussed in U.S. patent application Ser. No. 10/208,631 (Tyler). Tyler discloses tubular devices that control erosion by inhibiting the flow of water and lowering its velocity in order to allow solids to fall out of suspension. Tyler discloses filling the tubular erosion control devices with permeable reactive media to treat liquid runoff and separately discloses stacking the devices to form a retaining wall to control erosion. There are numerous reasons, however, why a barrier constructed from a stack of the tubular erosion control devices in Tyler would be ill-suited for treatment of liquid runoff.
First, a barrier constructed out of stacked tubes would necessarily have a pyramid shape and, therefore, a sloped face. Liquid runoff takes the path of least resistance and would tend to flow over the top of the pyramid-shaped barrier rather than through the tubes at the base of the structure. This would result in a significant amount of unfiltered runoff. This also means that the tubes at the base and in the middle of the pyramid do not actually filter a significant amount of liquid runoff whereas the tubes on the outside of the pyramid encounter more runoff than they can adequately filter. This imbalance results in high maintenance costs for the outer tubes, low efficiency and waste from the unused media in the inner tubes, and untreated runoff.
Second, accessing individual tubes in a barrier constructed according to Tyler in order to change or clean the reactive media in the tubes or to otherwise service the tubular devices would be costly and time consuming. Because most of the tubes in a pyramid-shaped barrier rely on other tubes for support, it would be necessary to destroy the entire barrier when even one tube near the base of the structure needed to removed for maintenance or modification.
Third, a barrier constructed out of tubes would have significant inter-container voids because tubes lack sides that can mate snugly with adjacent tubes. These voids would reduce the amount of contact between the reactive media in the tubes and the undesired solutes in the runoff, resulting in further untreated liquid runoff.
Finally, tubular erosion devices tend to roll when placed in the path of rapidly-flowing runoff. This can result in an unstable barrier that requires extensive maintenance and an elaborate and costly system for securing the tubes in the barrier to each other and securing the entire structure to the ground.
What is needed is a system for treatment of liquid runoff that uses reactive media to immobilize solutes of concern, that is easily assembled, maintained, modified, and augmented, and that has a geometry that enhances contact between the reactive media and the solutes.